Add replay button to anniversary video.

Buttons stolen from https://www.mozilla.org/en-US/firefox/independent/
Up until very recently, we had nowhere to host static assets being loaded by snippets, so this was the norm. Since snippets are downloaded once a day, caching per-asset isn't terribly important unless it has a large filesize (like the tail animation or video that's loaded separately later on in this snippet), and keeping them as data URIs within the snippet allows for us to send out a new snippet with an updated asset (like including a new icon in this font) without having to mess with hosting multiple versions of the asset simultaneously.